The invention relates to a cutting device for separating individual laminated chip assemblies from a strip of such assemblies, method of separation and a method of making the cutting device. More particularly the invention relates to cutting devices of the type used for cutting, dicing, slicing, and singulating of components used by the electronics industry.
There is a need for a cutting device for use in the electronics industry. Examples of such cutting operations are the dicing, slicing, and singulating which are involved in the manufacture or processing of silicon wafers and so-call pucks made of an alumina-titanium carbide composite, for the electronics industry in general and the computer industry in particular. As is well known, silicon wafers are processed for integrated circuits while alumina-titanium carbide pucks are utilized to fabricate flying thin film heads for writing (recording) and reading (playing back) information magnetically stored in computers.
Integrated circuits are typically assembled into a package which is mounted on a printed circuit board. There are many types of integrated circuit packages including quad flat pack (QFP), ceramic pin grid array (PGA) and ball grid array (BGA) and quad flat no lead (QFN) packages.
QFP packages contain a metal lead frame which has a plurality of individual leads which extend from a center die paddle. The die paddle supports the integrated circuit. The integrated circuit has a number of bond pads that are wire bonded to the individual leads. The integrated circuit and a portion of the lead frame are encapsulated in an injection molded plastic housing.
PGA packages have a plurality of pins that extend from a housing which encloses an integrated circuit. The pins of the package are mounted to an external printed circuit board. PGA package housings are typically constructed from a ceramic material, or a plastic/printed circuit boards, which create relatively high pin densities.
BGA packages have a plurality of solder balls that are attached to a substrate. The substrate may be constructed from ceramic or printed circuit boards. BGA chips are used in non-memory devices such as processors and flash memory units.
QFN packages comprise a printed circuit on a relatively thick conductive layer, typically copper, and are useful in memory applications. In the context we refer to Japanese patent publications JP-2001-53033 and JP-2001-77129.
Laminates of soft materials such as copper cannot easily be sliced cleanly without smearing and creating burrs along the edges of the chips. Such irregularities will interfere with the electrical contacts to the chip because the copper forms the chip connectors. If the chip connector is not of the right form then it must be rejected which drives up the rejection rate and accordingly the manufacturing costs. A QFN assembly has a thick copper layer for improved thermal conductivity and it is the most difficult category of chip assemblies to slice cleanly.
It has been realised that abrasive wheels may be used as cutting tools in the electronics industry. The use of abrasive wheels to cut silicon wafers and alumina-titanium carbide pucks is explained in U.S. Pat. No. 5,313,742, the entire disclosure of which patent is incorporated herein by reference. Ideally, cutting blades to effect such cuts should be as stiff as possible and as thin and flat as practical because the thinner and flatter the blade, the less kerf waste produced and the stiffer the blade, the more straight it will cut. However, these characteristics are in conflict because the thinner the blade, the less rigid it becomes.
Cutting blades are made up basically of abrasive grains and a bond which holds the abrasive grains in the desired shape.
It is known to use monolithic abrasive wheels, usually ganged together on an arbor. Individual wheels in the gang are axially separated from each other by spacers. Traditionally, the individual wheels have a uniform axial dimension from the wheel""s arbor hole to its periphery. Although quite thin, the axial dimension of these wheels is greater than desired to provide adequate stiffness for good accuracy of cut. However, to keep waste generation within acceptable bounds, the thickness is reduced. This diminishes rigidity of the wheel to less than the ideal.
It is an object of this invention to provide a cutting tool useful in separating laminated chip assemblies, especially for QFN assemblies having a relatively thick copper layer, i.e., 200-400 microns thick. A typical QFN assembly is 1200 microns thick, of which the copper layer is 220 microns thick.
It is another object of this invention to provide methods of separating a laminated chip assemblies, especially those having a relatively thick copper layer.
It is another object of the invention to provide a method of making a cutting tool for use in the method.
According to the invention in one aspect there is provided an abrasive wheel for use in separating one quad flat, no-lead, integrated circuit package from another by cutting through a bridging element which joins them, each package comprising a printed circuit board in a protective body therefor, the abrasive wheel comprising a disc formed of abrasive material comprising abrasive particles bonded in a matrix comprising cured polyimide resin and metal particles, the disc forming a cutting edge having a maximum thickness of the order of 350 micron, whereby the cut is substantially straight and is substantially free of smears and burrs.
According to the invention in another aspect there is provided cutting apparatus for use in separating one quad flat, no-lead, integrated circuit package from another by cutting through a bridging element which joins them, the apparatus comprising a plurality of abrasive wheels, each wheel comprising a disc formed of abrasive material comprising abrasive particles bonded in a matrix comprising cured polyimide resin and metal particles, the disc forming a cutting edge having a maximum thickness of the order of 350 micron, the wheels being mounted in parallel on a common shaft, whereby cuts in parallel bridging elements may be made when the shaft is rotated.
According to the invention in yet another aspect there is provided a method of separating one quad flat, no-lead, integrated circuit package from another by cutting through a metal bridge elements joining adjacent packages, each package comprising a printed circuit board in a protective body therefor, the method comprising rotating an abrasive wheel to cut through a bridge element, the wheel having a maximum cutting edge having a thickness of the order of 350 micron and formed of bonded abrasive particles in a matrix comprising cured polyimide resin, and metal particles, whereby the cut is substantially straight and little or no smears or burrs are formed on the cut surface.
According to the invention in yet another aspect there is provided a method of separating one quad flat, no-lead, integrated circuit package from another by cutting through a metal bridge element by which one package is joined to the other, each package comprising a printed circuit body in a protective body, the method comprising rotating an abrasive wheel having a cutting edge to cut through the bridging element, the edge having a maximum thickness of the order of 350 micron and being formed of abrasive particles held in a matrix of a hardened resin including metal particles and having a high glass transition temperature, whereby the cut is substantially straight and the cut surface is substantially free of smears and burrs.
According to the invention in yet another aspect there is provided a method of making an abrasive wheel having a cutting edge adapted to cut through a bridging element joining two integrated circuit packages and to leave a substantially straight cut substantially free of smears and burrs, the method comprising subjecting to temperature and pressure a composition comprising abrasive particles, metal particles and of a polyimide resin to cause the formation of a matrix having a high glass transition temperature containing the abrasive particles and the metal particles to weld together within the matrix, in a mould shaped to form an annulus defining a cutting edge having a maximum thickness of the order of 350 micron.